1. Field of the Invention
The present invention relates to a strike-type shutter device that is suitable for a simple camera like a lens-fitted photo film unit, more particularly relates to a shutter device that realizes high shutter speed with a simple structure.
2. Explanations of the Prior Arts
A strike-type shutter device is used for a lens-fitted photo film unit or a simple camera. The shutter device consists of a shutter blade which is normally at a closed position for closing an exposure aperture by bias of a spring, and a shutter drive lever which moves from a charged position to a release position in cooperation with shutter release. The shutter drive lever strikes the shutter blade to open the exposure aperture for a fixed period while its moving from the charged position to the release position by a bias of a spring. Then, the shutter blade is returned to the closed position by the bias of the spring, so that an exposure to the photo film is completed.
According to the shutter device, due to the small number of necessary component parts, it is possible to keep manufacturing cost low. It is also possible to obtain single shutter speed stably within a range of {fraction (1/100)}xcx9c{fraction (1/150)} seconds by choosing the coefficients of springs to urge the shutter drive lever and the shutter blade.
In case a high sensitivity photo film of ISO sensitivity 800 or 1600, however, photography under fine weather causes overexposure. As a countermeasure for over-exposure, it is proposed to use small diaphragm. Yet, keeping distance of flashlight reach unchanged upon flash photography loses special merits for using the high sensitivity photo film. As another countermeasure, it is proposed to shorten the shutter speed. For such a shutter which swings a single shutter blade for one exposure, however, the shutter speed of {fraction (1/150)} second or so was the limit to realize stable shutter speed.
An object of the present invention is to provide a shutter device for camera to realize a high shutter speed stably without changing the conventional method which strikes the shutter blade while moving the shutter drive lever in one direction.
Another object of the present invention is to provide a shutter device with high shutter speed at a low cost by keeping the number of component parts small.
To attain the above object and advantages, the shutter device of the present invention has a first blade held at a first position to close the exposure aperture by the bias of spring, and a second blade held at a second position to open the exposure aperture by the bias of spring. The shutter drive lever strikes the first blade and the second blade successively. The second blade begins to close the exposure aperture after the first blade opens the exposure aperture completely. After the exposure aperture is closed, the first blade and the second blade are overlapped with each other and rotate back to the primary positions with the exposure aperture closed. Owing to this, it is possible to realize a shutter device that can release a shutter stably and rapidly regardless of simple structure.
The rotation of the first blade is limited by a stopper. While the second blade is moving against the bias of the spring, the first blade is urged to return to the first position by the bias of the spring. The second blade overlaps with the first blade through the cutout formed in the first blade. The second blade strikes the first blade while closing the exposure aperture so as to limit its rotation.
The second blade is movable to a third position which is out of a moving range of the shutter drive lever. When the second blade is at the third position, the shutter drive lever strikes only the first blade, so that an exposure is carried out at a low shutter speed. Owing to this, it is possible to switch the shutter speed in two grades. When the second blade is at the second position, the shutter speed is determined in a range of {fraction (1/250)} to {fraction (1/500)} second. Meanwhile, the second blade is at the third position, the shutter speed is determined in a range of {fraction (1/100)} to {fraction (1/150)} second.